Retired US Marines general John Kelly is a battle-hardened commander who would bring a background of military discipline and order to Donald Trump's stormy White House as its new chief of staff.

Mr Kelly's experience as homeland security secretary and a veteran of three tours in Iraq, plus a sobering family tragedy, suggests he will be a loyal manager for the US president when he starts the job on Monday.

"He has been a true star of my administration," the president tweeted on Friday, announcing that his current secretary of homeland security was in - and White House chief of staff Reince Priebus out.

Mr Trump called Mr Kelly a "great leader" and "great American". He called Mr Priebus, ousted after a tumultuous six months, a "good man".

As homeland security secretary, Mr Kelly has taken the lead on some of Mr Trump's most controversial policies, including his executive orders suspending the admission of refugees and temporarily barring visitors from several Muslim-majority nations.

Those orders have been stripped down by courts pending a Supreme Court review in the autumn.

People who know Mr Kelly said he was not aware of the details of the initial orders until around the time Mr Trump signed it.

Yet, just days after taking office, he had to lead the agency as it dealt with the chaos and confusion that ensued at airports in the US and around the world.

He defended the orders to reporters and politicians and insisted he indeed had been part of the decision-making process.

Mr Kelly has stood up to Congress, another facet of his history that Mr Trump might find attractive.

In April, Mr Kelly bluntly challenged members of Congress critical of the Trump administration's aggressive approach to immigration enforcement to either change the laws or "shut up".

But Mr Kelly has won cross-party respect as a result of his distinguished military career.

He joined the Marine Corps in 1970, carving out a reputation as a highly respected but often outspoken commander who could stir debate and issue unpopular directives on issues ranging from women in combat to the treatment of detainees at the Guantanamo Bay, Cuba, detention centre.

Kelly was the fifth person to lead the Department of Homeland Security, which includes agencies that protect the president, respond to disasters, enforce immigration laws, protect America's coastlines and secure air travel.

His selection as secretary bolstered concerns about an increase in military influence in a Trump White House.

Mr Kelly also holds a sombre distinction as the highest ranking officer to lose a child in combat in Iraq or Afghanistan.

His son, Marine 1st Lt Robert Kelly, was killed in November 2010 in Afghanistan.

Mr Kelly retired from the military last year, wrapping up a three-year post as head of US Southern Command, which spanned some of the more fractious debate over the Obama administration's ultimately failed attempt to close Guantanamo.

In his final Pentagon news conference, Mr Kelly spoke about the loss of his son - a topic he did not often discuss publicly.

"To lose a child is ... I can't imagine anything worse than that. I used to think, when I'd go to all of my trips up to Bethesda, Walter Reed, I'll go to the funerals with the secretaries of defence, that I could somehow imagine what it would be like," he said.

But, he added: "When you lose one in combat, there's a - in my opinion - there's a pride that goes with it, that he didn't have to be there doing what he was doing.

"He wanted to be there. He volunteered."

Mr Kelly said he received "occasional letters from Gold Star families asking, 'Was it worth it?'

"And I always go back with this: It doesn't matter. That's not our question to ask as parents," he said.

"That young person thought it was worth it and that's the only opinion that counts."